Helical coil variable potentiometer and method of constructing same



. Feb. 24, 1970 w M E ET AL 3,497,855

HELICAL com VARIABLE POTEN'IIOMETER AND METHOD OF GONS'IRUCTING SAME Filed Sept. 18, 1967 2 Sheets-Sheet 1 INVENTORS 51/EN Tie- /44 Mun/.52 By /-/4M5 HABE/QEDEE 4770a NEV Feb. 24, 1970 MUELLER ET AL 3,497,855

HELICAL COIL VARIABLE POTENTIOMETER AND METHOD OF CONSTRUCTING SAME Filed Sept. 18. 1967 2 Sheets-Sheet 2 Jr rue/var United States Patent U.S. Cl. 338-143 6 Claims ABSTRACT OF DISCLOSURE A miniaturized helical coil type electric potentiometer made by splitting the internally helically grooved housing into halves and then assembling about a preformed helical coil of resistance wire.

BACKGROUND OF THE INVENTION This invention relates generally to variable electrical potentiometers of the helical coil type with a movable electrical contact or brush that rubs on said coil from the inside thereof, and traverses said coil from end to end. More particularly, the invention is directed to meth ods of constructing such potentiometers and to new constructional features and combinations facilitating the manufacture of a superior potentiometer of this type in a miniaturized size, and at comparatively low cost.

The conventional helical coil potentiometer has a hollow insulation cylinder formed interiorly with a helical groove, in which is positioned a helical coil of resistance wire. A brush on a slider inside the coil contacts the inner surface of this coil, while the slider meshes With the helical groove. The slider slides along a rotor which is rotated by an axial shaft, and accordingly both rotate and advance axially in such a manner that the brush which it carries slides along the helical coil. Potentiometers of this type have been used successfully in large numbers. However, their fabrication, involving as it does the step of providing a helical groove on the interior of a cylinder and then the installation of a helical coil thereon, is a ditficult operation, and particularly so in the case of a miniaturized scale.

SUMMARY OF THE INVENTION The present invention, briefly summarized, involves the concept of splitting the interiorly grooved housing cylinder into two (or more) parts, and assembling the parts about a pre-formed resistance wire coil. The housing has cylinder ends, and the housing parts are held in assembly about the coil by means of two end rings fitted tightly over these ends. These end rings are actually made to serve three functions: first, to hold the split housing together in assembly with the coil; second, to furnish integral external electrical connection tabs; and, third, to act as a heat sink, guarding against undue heating of and damage to the parent material of the housing during soldering of external electrical connections to the ring tabs.

BRIEF DESCRIPTION OF THE DRAWINGS The invention has additional features that will become evident in the course of the following detailed description of a present electrical embodiment thereof, reference for this purpose being had to the accompanying drawings, in which:

FIG. 1 is a perspective view, to approximately full scale, of a potentiometer in accordance with the invention;

FIG. 2 is a longitudinal medial section taken on line 2-2 of FIG. 1;

FIG. 3 is a transverse section taken on line 33 of FIG. 2;

3,497,855 Patented Feb. 24, 1970 '"ice FIG. 3a is a detail section on line 3a-3a of FIG. 3;

FIG. 3b is a section on line 3b3b of FIG. 3a;

FIG. 4 is an enlarged fragmentary detail taken from FIG. 2;

FIG. 5 is a detail section taken on line 5-5 of FIG. 1;

FIG. 6 is a view similar to FIG. 5 but showing the parts after finishing; FIG. 7 is a transverse section taken in accordance with line 77 of FIG. 2;

FIG. 8 is a section taken in accordance with line 8-8 of FIG. 2;

FIG. 8a is a section on line 8a8a of FIG. 2;

FIG. 9 is a perspective view of an electrical brush;

FIG. 10 is a perspective view of a slip ring; and

FIG. 11 is an exploded view of the potentiometer.

DESCRIPTION OF PREFERRED EMBODIMENT The potentiometer of the invention may be constructed in various sizes, but that principally in present contemplation is as shown approximately to full scale in FIG. 1.

The potentiometer has a housing 15 comprised of a cylinder or barrel 16, and two end caps 17 and 18 fitted to opposite ends of the latter. These body parts are preferably molded from a suitable thermoplastic material, such as a plastic marketed by Union Carbide Corporation, and identified as polysulfone, type P1710. The barrel 15 has molded therein the helical groove 20, and is split diametrically into two halves 20a and 20b. These halves have mating ribs and grooves 21 and 22, respectively, which aid in assembly. Thus when the two halves are in assembly, they form the continuous helical groove 20 adapted to snugly receive a resistance wire coil 24, which is preformed to fit precisely in the groove 20, in the very bottom thereof, as shown. The groove 20 is defined by a helical rib 26, and there is sufficient space in the groove 20, inside the coil 24, to receive helical thread elements 29 on opposite ends of a slider 30. The slider 30 has an aperture 32 through it which is in the general cross-sectional shape of a somewhat flattened hexagon; and received in this aperture, with a relatively close but free sliding fit, is an elongated rotor 34 which is tightly mounted on the end of an axially disposed rotatable potentiometer adjustment shaft 36. The slider and rotor are preferably molded from the same material as the housing sleeve.

The housing cylinder or barrel 16 formed by the two housing halves 20a and 20b has, projecting from each end, on an outside diameter which is somewhat smaller than the barrel, an annular end ring or projection 38, and on each of these is fitted a flat, electrically conductive holding and electric terminal ring 40', shouldered tight against the barrel shoulder 41. In the assembly of the potentiometer, the wire coil is first preformed to its final shape or form, then nested into the half-helical groove in one of the housing parts 20a, 20b, and the other housing part then fitted over the first, the coil being at this time nicely and snugly received in the half-helical groove in said other housing part. The two housing parts are then moved together until they meet along the plane of their split, with the coil worked down in the helical groove, and the rings 40 are then installed, holding the parts described in tight assembly.

Each ring 40' has a radially projecting electrical terminal tab 45, and inside and in line therewith, the barrel end ring part or projection 38 is notched, as at 46, and to one side of this notch is apertured through the first turn of the helical rib 26, as at 47, so as to expose the resistance wire of the coil at a point near its end, in the last turn of the rib. A short length of metallic ribbon 48 is then spot welded to the ring 40, in line with the tab 45, and is spot welded also to the resistance wire coil where exposed by the aperture 47. The two ends of the coil are thus electrically connected to the two terminal tabs 45. A feature of the invention is that in soldering external electrical connections to the tabs 45, the heat from the operation flows into the rings 40, which are of enough bulk to serve adequately as heat sinks, preventing temperature rises such as would otherwise tend to damage the thermoplastic material of the potentiometer housing. This feature is deemed to be of material importance, in that it solves an otherwise difficult problem.

The aforementioned end caps 17 and 18 of housing comprise end walls 50 and 51, and annular flanges 5.2 and 53, respectively. The annular flanges 52 and 53 are shaped to form depressed interior annular seats 60, which engage the aforementioned metal rings 40 mounted on the end ring portions 38 of the barrel, these flanges having a slip-fit over the metal holding rings 40 as well as over the extremities of the end ring portions 38 on the barrel. Preferably, these end ring portions 38 are formed on the exterior thereof with a circumferential groove 64, and the caps 51 and 52 are provided with a series of small ribs r adapted to snap into the groove 64 when the caps are pressed home so as to act as locks. The extremities 38 of the barrel will be understood to seat against the confronting end wall portions 50 and 51 of the end caps, as clearly shown in the drawings. The end ring portions 38 of the two barrel halves a and 20b are each provided with a pair of pins 66, which project with close fit through corresponding apertures 67 in the end walls of the caps in the assembly of the housing. The outer ends of the apertures 67 are countersunk conically, as indicated at 68, and, initially, the pins 66 project beyond the outer surface of the caps to an extent such as illustrated in FIG. 5. To seal the assembled housing, the pins 66 are heated, as by touching them with a suitably shaped hot iron, and so caused to melt and to fill in the conical countersinks 68. The defining surfaces of the latter also soften and melt from the environmental heat, and the material of the pin '66 thus both fills the countersink, and fuses to the defining surfaces of the latter. The initial lengths of the pins 66 are made such that the countersinks are exactly filled, resulting in a flush finished exterior surface as appears in FIG. 6, with the exception that the junction line between the pin and surrounding material has disappeared owing to the fusion.

The rims of the caps 17 and 18 are notched, as at 69, to receive and pass the ring terminal tabs 45.

The end cap 18 has molded thereon one end of a panel mounting bushing 70- that surrounds the potentiometer shaft 36. This bushing 70 will be further referred to hereinafter. The shaft rotor 34 has on its axial extremity, at the end opposite from shaft 36, an axial stub shaft 72, which is rotatably received in a socket 74 molded in end cap 17.

Sunk to a flush position in a slot in the inside surface of the end wall 50 of cap 17 is an electrical slip ring 80 and its radial terminal tab 82, the latter projecting through a slot in the rim of cap 17, as clearly shown.

The rotor 34 has, extending longitudinally along each of its two narrow opposite faces, a dove-tail groove 90, in which is confined the angular shank 91 of an electrical conductor 92 (FIGS. 3, 10, and 11). The apex of this shank projects slightly outside the groove 90, and is contacted by a presently described contact member in all positions of the slider along the rotor. A single groove 90 could be provided, and the second, as here shown, is a spare. The extremity of shank 91 carries a yoke 93 bearing two spherically rounded contacts 94 'which ride on slip ring 80. The yoke arms are so formed as to be slightly deflected and to exert light spring pressure on the slip rings to assure good electrical contact.

In each end cap is formed a spiral ramp 100 whose high end portion is engageable, as at 101, by an edge 102 of the slider to afford an end stop for the slider.

Referring again to the bushing 70 (FIG. 2), it will be observed that there is a clearance between the bushing and the shaft 36, and that this is made possible by forming the end cap 18 with a bearing portion 18a for the shaft. Thus, a cheap material, such as aluminum, can be used for both the bushing and the shaft. It is of course understood that an aluminum shaft, or bearing bushing, working against a metal part, results in galling and undue wear. On the other hand, an aluminum shaft in a plastic bearing is entirely satisfactory. The construction of the invention thus permits use of a cheap aluminum shaft, though of course brass or other materials can be used if desired.

An electrical brush or contact 94 (FIGS. 3, 3a, 3b, 9 and 11) is mounted on the rotor 34 to make continuous contact between selected points on the coil 24 and the rotor conductor 92. This contact element, which is composed of a resilient electrically conductive metal, comprises a shank 96, with one end portion or arm 97 bent upwardly at a sharp angle (as seen in FIGS. 3 and 9), and formed with a convex tip 98 adapted to contact and ride smoothly along the resistance wire coil. The opposite extremity of the shank 96 is bifurcated, as illustrated, affording two flexible and resilient contact strips. At one side of the shank 96 is a generally box-shaped formation 99, which is adapted for insertion and retention in an aperture 100 of generally similar cross-section formed in the slider 30. The shank extends through suitable clearance space between the slider and rotor so that its flexible bifurcated ends ride on the ridge of the conductor stem 91. The arm 96 bears on the coil, as already described.

The operation of the potentiometer will now be understood by those familiar with and skilled in the art. The novel manufacturing approach and constructional features of the invention, however, bring about simplifications which speed up the manufacture of the potentiometer, and at the same time very materially reduce its cost. At the same time, neither excellence of performance nor long life has been sacrificed.

The present drawings and description are of course of one present illustrative embodiment, and it will be understood that various changes in design structure and arrangement may be made without departing from the spirit and scope of the appended claims.

We claim:

1. In a helical coil potentiometer, an external housing including a plastic body having an axial cylindrical bore with a helical groove formed therein, said body being split into a plurality of parts on planes radially disposed relative to the axis of said bore, a preformed resistance wire coil seated in said helical groove, annular axially directed projections on opposite ends of said body, metallic electrically conductive rings tightly fitted onto said projections for holding said body parts in assembly about said coil, with said coil seated in said helical groove, said rings including electrical terminal tabs extending externally of said housing, and electrical connections between the end portions of said coil and said rings.

2. A potentiometer as in claim 1 wherein said external housing further includes molded plastic end caps comprising end walls and annular flanges adapted for slipfit connection over said annular body projections and said rings.

3. A potentiometer as in claim 2 together with means for securing said end caps to said housing comprising integral pins formed on and extending from said annular projections into said end caps and fused integrally thereto.

4. A potentiometer as in claim 2 together with axial shaft means journalled in said'end caps, a rotor on said shaft, a slider longitudinally movable along said rotor and axially of said shaft, said slider including means engaging said helical groove and guided thereby, an electrical contact element mounted on said slider in such manner as to bear on said coil, a conductor mounted on said rotor and extending longitudinally thereof in such mannor that it continuously contacts said contact element as said slider is moved longitudinally of said rotor, a slip ring mounted on the inside Wall of one of said end caps, and a contact element extending from said conductor and bearing on said slip ring.

5. A potentiometer as in claim 4 wherein a plastic bearing is formed in one of said plastic end caps, and said shaft means is journalled in said plastic bearing.

6. A potentiometer as in claim 5 together with a panel mounting bushing aflixed to said end cap, said bushing having an internal diameter greater than the diameter of said shaft means to provide clearance between said shaft means and said mounting bushing.

References Cited UNITED STATES PATENTS 2,495,321 1/1950 Gibbs et a1. 338-269 X 2,859,316 11/1958 Miller 338-148 2/1962 Perier et a1. 338l48 4/1962 Mucher 338143 10/1962 Laubenfels 338143 11/1962 Gamble 338261 4/1967 Kruse 338184 X 7/1968 Doering 338184 FOREIGN PATENTS 2/ 1967 Austria.

LEWIS H. MYERS, Primary Examiner US. Cl. X.R. 

